1. Field of the Invention
The present invention relates to an apparatus for selectively exposing the peripheral portion of a circular substrate such as semiconductor wafer for use to manufacture a semiconductor element.
2. Related Background Art
A problem has been experienced with a photoresist, which is applied to a circular substrate such as a wafer having a cut portion (an orientation flat or notch) Of a predetermined shape, in that it can easily be separated at the peripheral portion of the wafer and the separated photoresist adheres to the surface of the wafer as a foreign matter, causing an undesirable influence to be exerted upon the lithographic process included in the process of manufacturing a semiconductor. The separation of the photoresist can be effectively prevented by exposing it so as to harden it. Accordingly, an exclusive exposing apparatus has been used in which the wafer is rotated while irradiating the peripheral portion of the wafer with exposing light so that a predetermined exposure width (about 1 to 7 mm) of the peripheral portion of the wafer is selectively exposed. A specific structure has been disclosed in, for example, Japanese Patent Laid-Open No. 2-56924 (related to U.S. patent application Ser. No. 396,557 filed on Aug. 21, 1989). According to this disclosure, the structure comprises a light emitting portion (for example, an optical fiber) disposed extremely adjacently to the peripheral portion and capable of emitting exposing light, to which the resist is satisfactorily sensitive to the resist, and a light receiving portion disposed to confront the light emitting portion while holding the peripheral portion of the wafer, wherein the light emitting portion and the light receiving portion are integrally formed in such a manner that the integrated unit can be relatively moved in the radial direction of the wafer. When the peripheral portion is exposed to light, a portion, which corresponds to the predetermined exposure width (the radial directional length of the wafer), of the exposing light beams emitted from the light emitting portion is shielded by the wafer. The light beams which have not been shielded by the wafer are received by the light receiving portion. A signal obtained from the light receiving portion is used in the servo operation for maintaining the exposure width of the wafer at a predetermined value. That is, in accordance with the change in the level of the signals supplied from the light receiving portion, the light emitting portion, the light receiving portion and the wafer are respectively relatively moved in the radial direction of the rotation of the wafer so that a control is performed in such a manner that the peripheral portion is always exposed to light for a predetermined exposure width.
However, the above-described conventional technology is arranged in such a manner that the signal obtained from the light receiving portion is used to perform the servo operation. Therefore, when the intensity of the exposing light is excessively lowered due to the deterioration in the light source or when the light source is interchanged, the quantity of the light received by the light receiving portion can be considerably changed from a desired quantity with respect to the same exposure width. Therefore, the establishment of the correspondence from the aiming value for the servo operation cannot be made clearly. Therefore, a problem arises in that the exposure width at the time of the periphery exposure cannot be stabilized. Furthermore, if the intensity of the exposing light beam in the radial direction is not distributed uniformly, the relationship of the level of the signal in the light receiving portion with respect to the set exposure width cannot be made to be in the linear form. Therefore, it has been difficult to control the exposure width only by the light receipt signal obtained at the time of exposing the peripheral portion of the wafer. In addition, the servo conditions for the orientation flat (hereinafter called an "OF") is more strict than the circumferential portion. Therefore, the exposure width in this portion cannot be made uniform.
The above-described problems can be overcome by:
(1) An optical system for making uniform the intensity distribution of the exposing light beam is provided in the exposing light irradiating system.
(2) The levels of the light receipt signals which correspond to the set exposure width are respectively stored in a memory table.
However, in the case of (1), an excessively large optical system must be provided so as to make sufficiently uniform the intensity distribution. In the case of (2), a table of an excessively large memory capacity must be provided if the step, in which the exposure width in the periphery portion can be set, is desired to be finely sectioned. Furthermore, the table must be made again whenever the intensity distribution of the exposing light beam has been changed. If the signal value is obtained by using an approximate expression, the error of the approximate expression adversely effects the accuracy of the exposure width.